I've been running a Bitcoin-qt client (v0.7.2 beta) on my Mac for over 24 hours, with blockchain fully sync'ed.
I checked http://blockchain.info/connected-nodes at various times for my IP address. In the first few hours my IP appeared in the list. Then it disappeared for maybe 10 or 15 hours. Currently, it's back in the list, but showing a connection time of only 62 minutes, despite my network connection and IP address being completely stable, and a steady stream of inbound & outbound network traffic to/from the Bitcoin-qt client during the entire >28 hour period since I started it up.
Why would my IP show up at http://blockchain.info/connected-nodes for a certain time, then disappear from the list, then re-appear?
(It also appears that, as of this writing (2013-02-19 18:02:30 UTC,) no client on the list has a connection time of more than about 340-360 minutes. I'd expect it to be in the thousands to tens of thousands of minutes for at least some clients.)